Recent increases in the degree of integration and density in electronic parts have increased the power consumption per chip, and therefore, an important problem has arisen of how efficiently the generated heat is radiated or how effectively the elevation of the temperature is suppressed. Accordingly, the development of a material having an excellent thermal conductivity, which is suitable as an insulating sealing material for semiconductors, a material for a substrate on which parts are mounted, and a peripheral material such as a heat-radiating spacer, is demanded.
Another problem concerning heat is how to best maintain the conformity of the thermal expansion coefficient of an element and of an insulating material for sealing the element. If there is a difference in the thermal expansion coefficient, the thermal stress is repeatedly applied to the bonding portion by the heat cycle when the operation of an integrated circuit (IC) is stopped with the result that the element is damaged. In general, since a resin has a high thermal expansion coefficient, an inorganic filler having a low thermal expansion coefficient is incorporated into the resin to match the thermal expansion coefficient with that of the element.
For example, an epoxy resin containing about 70% by weight of fused silica is generally used as an insulating sealing material for a large-scale integrated circuit (LSI), because of a low thermal expansion coefficient and a good conformability. However, since fused silica has an extremely low thermal conductivity, this epoxy resin material is not suitable for use in a field where heat radiation is very important. Accordingly, a composition having a thermal conductivity of about 60.times.10.sup.-4 cal/cm.multidot.sec.multidot..degree.C., which is formed by filling a large amount of crystalline silica having a high thermal conductivity, is already in practical use. However, crystalline silica having a high thermal conductivity has a defect of a high thermal expansion coefficient. Therefore, the above two problems concerning heat, that is, the problems of heat radiation and thermal expansion conformation, cannot be solved by the use of fused silica or crystalline silica alone.
When fused silica and crystalline silica are simultaneously incorporated, since crystalline silica has a high Mohs hardness and comprises pulverized particles having irregular sharp cutting edges, if the amount of crystalline silica incorporated is increased, the flowability of the composition is drastically reduced and the wear of a kneader or a forming mold is greatly increased, and therefore, the amount of crystalline silica incorporated is restricted. As a means of reducing the wear, a method can be mentioned in which fused silica having a specific particle size distribution and a low abrasive property is used as a coarse particle fraction and mixed with a fine particle fraction of crystalline silica having a specific particle size distribution, to provide a composition having an excellent flowability and a reduced wear of a mold, as disclosed in Japanese Unexamined Patent Publication No. 58-164250. However, even in this composition, a reduction of the thermal conductivity due to fused silica cannot be avoided.
At present, even in a sealing material having a high thermal conductivity, the thermal conductivity is 60.times.10.sup.-4 cal/cm.multidot.sec.multidot..degree.C. at highest, and a level of 100.times.10.sup.-4 cal/cm.multidot.sec.multidot..degree.C. desired as the next target cannot be reached by using crystalline silica. Accordingly, .alpha.-alumina, aluminum nitride, and silicon carbide are under consideration as the filler having a higher thermal conductivity than that of crystalline silica, and alumina is promising as the substitute for silica because alumina is relatively inexpensive, has a stable quality, and has a good general-purpose property.
However, in view of the characteristics of existing alumina, existing alumina cannot be considered suitable as a filler for rubbers or plastics. For example, alumina prepared according to the Bayer process consists of irregular-shaped or plate-like primary particles having a size of several .mu.m to about 10 .mu.m at largest. Since this alumina has a large oil absorption, the filling property in rubbers or plastics is poor and the amount filled is limited to about 80% by weight, such that if alumina is incorporated, for example, into an epoxy resin, the thermal conductivity is 60.times.10.sup.-4 cal/cm.multidot.sec.multidot..degree.C. at highest. It is known that alumina formed by pulverizing electrofused alumina or sintered alumina is usable as a material of an abrasive or refractory. Since alumina of this type consists of dense corundum particles and the particle size can be optionally adjusted within a broad range of from a fine particle size to a coarse particle size of scores of .mu.m, the oil absorption is small and the filling property in rubbers or plastics is excellent. However, the pulverized particles of alumina of this type have many sharp angles and corundum (.alpha.-Al.sub.2 O.sub.3) has a high Mohs hardness. Therefore, the abrasive property is larger than that of crystalline silica having the same particle size, and corundum has a defect in that bonding wires or semi-conductor elements are damaged thereby.
A rounded spherical shape having no cutting edges is desirable for alumina particles. As the process for preparing spherical alumina particles, a flame-spraying process is known in which alumina according to the Bayer process is sprayed into a high-temperature plasma or oxyhydrogen flame, and the alumina is fused and then rapidly cooled to form spherical particles. However, this process is economically disadvantageous in that the heat consumption is large, and although the obtained alumina is composed mainly of .alpha.-Al.sub.2 O.sub.3 it generally contains .alpha.-Al.sub.2 O.sub.3 as a subsidiary component. The presence of this subsidiary component is unpreferable because the thermal conductivity of alumina is decreased by the subsidiary component.
As a means for solving the foregoing problems of the conventional techniques, several processes have been proposed for preparing .alpha.-Al.sub.2 O.sub.3 (corundum) particles having a particle size larger than 5 .mu.m and a regular shape. For example, Japanese Examined Patent Publication No. 60-33763 discloses a process in which aluminum hydroxide having a high sodium content is preliminarily dehydrated, a specific mineralizing agent is added to the dehydration product, and the mixture is calcined in a rotary kiln to obtain coarse particles of alumina. Furthermore, Japanese Unexamined Patent Publication No. 58-181725 discloses a process in which a mineralizing agent containing fluorine and/or boron is added to dry-type absorption alumina and the mixture is calcined in a rotary kiln to obtain similar coarse particles of alumina. However, coarse particles of alumina prepared according to these processes have a shape including regular cutting edges, as shown in the drawing (microscope photograph) of Japanese Unexamined Patent Publication No. 58-181725, and do not possess a rounded spherical shape. Moreover, Japanese Unexamined Patent Publication No. 56-35494 (U.S. Pat. No. 4,307,147) discloses a substrate having a covering film composed of a dispersion of polygonal corundum particles in an organic polymer. These corundum particles, however, are angular and do not possess a rounded spherical shape.
It is considered that, if spherical corundum particles in which sharp cutting edges are eliminated to reduce the abrasive and wearing properties are used and incorporated as a filler in a resin, a rubber or plastic composition having a good filling property, a reduced abrasive and wearing property, an excellent thermal conductivity, and a high thermal conductivity optimal for an insulating sealing material or the like, will be obtained. However, the kind of the resin in which such spherical corundum particles are to be incorporated, the amount added of the particles, and the incorporation method have not been investigated in detail, and a rubber or plastic composition having a high thermal conductivity and practically usable as a sealing material has not yet been developed.